scsi: sr: support runtime pm

This patch adds runtime pm support for sr.

It did this by increasing the runtime usage_count of the device when
its block device is accessed. And decreasing the runtime usage_count
of the device when the access is done.

If there is media inside, runtime suspend is not allowed as we don't
always know if the ODD is being used or not.

The idea is discussed here:
http://thread.gmane.org/gmane.linux.acpi.devel/55243/focus=52703
and the restriction to check media inside is discussed here:
http://thread.gmane.org/gmane.linux.ide/53665/focus=58836

Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: James Bottomley <James.Bottomley@HansenPartnership.com>
libata: scsi: no poll when ODD is powered off

When the ODD is powered off, any action the user did to the ODD that
would generate a media event will trigger an ACPI interrupt, so the
poll for media event is no longer necessary. And the poll will also
cause a runtime status change, which will stop the ODD from staying in
powered off state, so the poll should better be stopped.

But since we don't have access to the gendisk structure in LLDs, here
comes the disk_events_disable_depth for scsi device. This field is a
hint set by LLDs to convey information to upper layer drivers. A value
of 0 means media poll is necessary for the device, while values above 0
means media poll is not needed and should better be skipped. So we can
increase its value when we are to power off the ODD in ATA layer and
decrease its value when the ODD is powered on, effectively silence the
media events poll.

Signed-off-by: Aaron Lu <aaron.lu@intel.com>
scsi: remove can_power_off flag from scsi_device

Commit 166a2967b45ede2e2e56f3ede3cd32053dc17812 "libata: tell scsi layer
device supports runtime power off" introduced the can_power_off flag for
scsi_device and is used to support ZPODD implementation in SCSI layer.
Since ZPODD is now implemented in ATA layer, that flag is no longer
needed, so remove it.

Signed-off-by: Aaron Lu <aaron.lu@intel.com>